This discussion paper is/has been under review for the journal Solid Earth (SE).Please refer to the corresponding final paper in SE if available.
Seismic LAB or LID? The Baltic ShieldcaseM. Grad1, T. Tiira2, S. Olsson3,*, and K. Komminaho2
1Institute of Geophysics, Faculty of Physics, University of Warsaw, Pasteura 7, 02-093Warsaw, Poland2Institute of Seismology, University of Helsinki, P.O. Box 68, 00014, Helsinki, Finland3Department of Earth Sciences, Uppsala University, Villavägen 16, 752 34 Uppsala, Sweden*now at: Department of Mineral Resources, Geological Survey of Sweden, P.O. Box 670,75128 Uppsala, Sweden
Received: 2 May 2013 – Accepted: 3 May 2013 – Published: 23 May 2013
The problem of the asthenosphere for old Precambrian cratons, including East Euro-pean Craton and its part – the Baltic Shield, is still discussed. To study the seismiclithosphere-asthenosphere boundary (LAB) beneath the Baltic Shield we used recordsof 9 local events with magnitudes in the range 2.7–5.9. The relatively big number of5
seismic stations in the Baltic Shield with a station spacing of 30–100 km permits forrelatively dense recordings, and is sufficient in lithospheric scale. For modelling of thelower lithosphere and asthenosphere, the original data were corrected for topographyand the Moho depth for each event and each station location, using a reference modelwith a 46 km thick crust. Observed P and S arrivals are significantly earlier than those10
predicted by the iasp91 model, which clearly indicates that lithospheric P and S ve-locities beneath the Baltic Shield are higher than in the global iasp91 model. For twonorthern events at Spitsbergen and Novaya Zemlya we observe a low velocity layer,60–70 km thick asthenosphere, and the LAB beneath Barents Sea was found at depthof about 200 km. Sections for other events show continous first arrivals of P waves with15
no evidence for “shadow zone” in the whole range of registration, which could be in-terpreted as absence of asthenosphere beneath the central part of the Baltic Shield,or that LAB in this area occurs deeper (> 200 km). The relatively thin low velocity layerfound beneath southern Sweden, 15 km below the Moho, could be interpreted as smallscale lithospheric inhomogeneities, rather than asthenosphere. Differentiation of the20
lid velocity beneath the Baltic Shield could be interpreted as regional inhomogeneity. Itcould also be interpreted as anisotropy of the Baltic Shield lithosphere, with fast velocityclose to the east-west direction, and slow velocity close to the south-north direction.
1 Introduction
In the new global plate tectonics the crucial elements are moving lithospheric plates.25
The lithosphere (from the Greek: λíθoς – rocky, rigid) is understood as an outer shell
of the Earth, which consists of the crust and a certain part of the upper mantle. Rigidlithosphere, sometimes called the “lid”, is underlain by ductile asthenosphere (fromthe Greek : αiσθενως – weak). Petrological studies of xenoliths and thermal modellingbased on heat flow measurements allow to determine temperature gradients in thelower lithosphere, give some image of lithosphere composition and provide important5
suggestions about the depth of the wide transitional zone between lithosphere andasthenosphere named as LAB (lithosphere-asthenosphere boundary). The LAB is nota sharp discontinuity, but rather a gradual and wide transition zone (see e.g. Meiss-ner, 1986). From the point of view of elastic properties and seismic velocities, theasthenosphere could be identified as a low-velocity channel in the 50–200 km depth10
range in Gutenberg’s global model of the Earth (Gutenberg, 1959). Later, low seismicvelocities in the upper mantle were found also in regional scale beneath continentsand oceans (e.g. Anderson and Toksöz, 1963; Johnson, 1967; Schubert et al., 1976;Grad, 1988). The LAB depth strongly depends on temperature: thicker lithosphere upto about 200 km is clearly observed under “cold” Precambrian shields and platforms,15
while the thinnest lithosphere of 50–100 km is found under “hot” oceans and oceanicand continental rifts. Recently the lithosphere-asthenosphere boundary depth and lowvelocities are effectively estimated from velocity variations of global and regional to-mography models, group velocity of long period (T ∼ 100 s) surface waves and S wavereceiver functions (e.g. Bruneton et al., 2004; Gregersen et al., 2006; Li et al., 2007;20
Pasyanos, 2010; Wilde-Piórko et al., 2010). The LAB may also correlate with a down-ward extinction of seismic anisotropy or a change in the anisotropy direction (e.g. Eatonet al., 2009; Plomerová and Babuška, 2010). The asthenosphere could be also iden-tified with a low-viscosity zone and low value of the quality factor QS (e.g. Stacey,1969). Characteristic for asthenosphere is low seismic activity or even total lack of25
earthquakes. The electrical LAB is marked by a significant reduction in electrical resis-tivity, where resistive lithosphere is overlying a highly conductive asthenosphere. Theelectrical asthenosphere, identified as a high conductive layer (low resistivity layer) inthe upper mantle often coincides with a seismic low-velocity zone (e.g. Eaton et al.,
2009; Martinec and Wolf, 2005; Korja, 2007). Last but not least is thermal lithosphere.The use of the borehole heat flow measurements allows calculation of the lithosphericgeotherms and estimation of the thermal lithosphere thickness, defined as the depthwhere the continental geotherms intersect the 1300 ◦C mantle adiabat (e.g. Artemieva,2007; Artemieva and Mooney, 2001).5
Because mostly of their temperature nature the both boundaries of the astheno-sphere: upper – LAB, and lower – bottom of the asthenosphere, are not first orderseismic discontinuities, but rather gradient zones. Because of this, a large contrast ofelastic parameters is not expected and we do not expect strong enough reflected andconverted phases – which is not good news for seismic reflection and receiver function10
techniques. So, as the most promising for the determination of the seismic LAB depthremain the methods of surface waves, seismic tomography, and searching for “shadowzones” of P and S body waves. Shadow zones are expected at epicentral distancesabout 1500–2000 km. For this, local seismic events of relatively big magnitudes permit-ting good quality records at long distances from the source could be used (Fig. 1).15
2 The Baltic Shield and previous seismic investigations of the lithosphere andLAB
The East European Craton (EEC) is the coherent Precambrian part of Europe, mainlyof Archaean and Palaeoproterozoic age, assembled in the Late Palaeoproterozoic(Bogdanova et al., 2005, 2006). In the south-west, EEC is limited by accreted Phanero-20
zoic terranes of the Trans-European suture zone (TESZ) which extends from the BritishIsles to the Black Sea region. The northeastern part of the European continent occu-pies the Baltic (also Fennoscandian) Shield with the Precambrian bedrock exposed onthe Earth’s surface (Figs. 1 and 2).
The typical three-layer crust of the Baltic Shield has a thickness in the range of 42–25
60 km, and attains its maximum thickness in Central Finland. The P wave velocitiesin the upper crust are 6.0–6.4 kms−1, in the middle crust 6.6–6.9 kms−1 and in the
lower crust> 7.0 kms−1. The thickness of the upper and middle crust is in the rangeof 32–42 km. The thickness of the lowermost high-velocity crustal layer is in the rangeof 4–24 km. The P wave velocity in the uppermost mantle is 7.9–8.2 kms−1. The ra-tio of P wave velocity to S wave velocity varies typically from 1.67–1.70 in the uppercrust to 1.77–1.80 in the lower crust. In the upper mantle the ratio is about 1.73 (e.g.5
Guggisberg, 1986; Grad and Luosto, 1987; Luosto, 1997; Hyvonen et al., 2007).Similar to other old Precambrian cratons, features of the asthenosphere beneath the
Baltic Shield are still discussed. First determinations of the lithospheric thickness be-neath the Baltic Shield were obtained from analyses of fundamental-mode and higher-order Rayleigh surface waves. The dispersion of higher-mode data have been inter-10
preted by Nolet (1977) to distinguish the thick lithosphere of the Baltic Shield fromthe thinner Western European lithosphere (see also Zielhuis and Nolet, 1994). Caraet al. (1980) using higher modes found no need for a low-velocity zone in the man-tle beneath northern Eurasia. They also argue that a nearly constant 4.5–4.6 kms−1
S wave velocity is required in the uppermost 200 km. Using Rayleigh-wave disper-15
sion data for the Fennoscandian region, Calcagnile (1982) found lid thicknesses upto around 135 km in the Bothnia – north-central Finland area with weak, if any, shearvelocity contrast to the underlying layer. The surrounding areas are characterized bylid thicknesses up to around 75 km only. A stronger low-velocity zone with lid contrast0.25/0.45 kms−1 may be found in the Caledonian and the Baltic Sea area (Calcagnile,20
1982). An updated map of the lithosphere-asthenosphere system in Europe (Panza,1985; Calcagnile and Panza, 1987) shows much larger lithospheric thickness of theBaltic Shield, in the range 110–170 km, increasing to > 190 km in its central part. TheBaltic Shield model obtained later by Dost (1990) shows an absence of the low-velocitylayer, while density seems to be lower in 200–350 km depth.25
The deep structure of the lower lithosphere and asthenosphere can also be inves-tigated from studies of P waves. From the explosive source refraction profile FEN-NOLORA Guggisberg (1986) interpreted several deep low-velocity channels (of about5 % decrease in P wave velocity in a few tens km thick layers) in the lower lithosphere,
with the bottom of the lithosphere at about 200 km depth marked by a velocity changeof a couple of per cent. Regional 1-D S wave velocity models obtained for the cen-tral part of the Baltic Shield from the SVEKALAPKO array (Kozlovskaya et al., 2008)show that velocities beneath craton are significantly bigger (about 4 % down to a depthof 250 km) compared to standard model iasp91 (Kennett and Engdahl, 1991). On the5
other hand lowering of the velocity with depth is not visible (Kozlovskaya et al., 2008).This fact could be interpreted as absence of asthenosphere, or that LAB in this areaoccurs deeper. Plomerová and Babuška (2010) define the LAB as a boundary betweena fossil anisotropy in the lithospheric mantle and an underlying seismic anisotropy re-lated to present-day flow in the asthenosphere. The LAB topography is more distinct10
beneath the Phanerozoic part of Europe than beneath its Precambrian part. Beneathcentral Fennoscandia the LAB deepens down to ∼ 220 km (Plomerová and Babuška,2010).
3 Seismic data for the Baltic Shield
The area of the Central and Northern Europe is a region characterized by weak to mod-15
erate seismicity (e.g. Slunga, 1991; Ahjos and Uski, 1992). The map in Fig. 1 showslocation of 2698 events in the Baltic Shield and surroundings, recorded from January2008 to September 2011. Most of these events were classified as earthquakes. Eventsrecorded at less than three stations, as well as those which were explosions fromknown sites, were omitted from the data set. Mining-induced seismic events (e.g. rock20
bursts, mine collapses) were also excluded from the cataloque. However, it is likely thatman-made events still exist in the data, especially among low magnitude events (Uskiand Raime, 2010).
It is seen from Fig. 1 that seismic events are concentrated in few distinct areas. Inthe north (18–20◦ E, 77–78◦ N) a group of events is concentrated in the southern part25
of Spitsbergen, within a continental crust of the Barents Sea block (see also Figs. 2and 3). West of 10◦ E and north of 73◦ N, a linear group of epicenters is related to the
Knipovich Ridge and Mohns Ridge segments of the Mid-Atlantic Ridge. Events south ofthis group are related with continent-ocean transition (COT), transition between oceaniccrust of the Atlantic and continental crust of the Barents Sea. The coastal area ofScandinavia is characterized by moderate seismicity of the Caledonides. In the southof the study area (6–8◦ E, 50–52◦ N), a group of events is related with seismic activity5
of young continental rift – Rhein Graben. Events scattering around 16◦ E and 52◦ N arerelated to the area of cupper mines activity in SW Poland (Lubin area). There are bothnatural and induced earthquakes in this area. Next to the east, the area around 19◦ Eand 50◦ N is related to the seismicity of coal mines activity in Silesia (Draber et al.,2002). The Baltic Shield in general is a stable intraplate region characterized by weak10
to moderate seismicity, with event magnitudes ML rarely exceeding 4.0. The majority ofearthquakes occur within the Kuusamo-Kandalaksha region in north-eastern Finlandand adjacent Russia (28–35◦ E, 65–68◦ N), and along a broad N-S oriented belt runningparallel to an ancient plate boundary, from the Bothnian Bay to northern Norway (from18◦ E, 63◦ N to 25◦ E, 70◦ N) (Ahjos and Uski, 1992). Events in the Kuusamo area are15
the most frequently recorded natural earthquakes in Finland. However they are ratherweak, and most of them are detected only because there is a local seismic array (Fig. 2)in this area. In the south of the Baltic Shield, an area of rare seismicity in Scania (11–14◦ E, 55–57◦ N) has had some relatively strong (with magnitude up to 5) events. In theFennoscandian area 80–90 % of all earthquakes occur in the upper 20 km of the Earth’s20
crust (Slunga, 1991; Ahjos and Uski, 1992). Earthquakes in the depths of 40 km anddeeper are rare. Some well-determined events in northern Sweden have been locatedclose to the crust-mantle boundary (Arvidsson and Kulhanek, 1994; Arvidsson, 1996).These observations suggest that the Archean lower crust may be seismically activeand involved in brittle deformation (Uski et al., 2012).25
To study of the lithosphere-asthenosphere system of the Fennoscandia we needrelatively effective sources (earthquakes or quarry blasts) generating seismic waveswhich could be recorded up to a distance of at least 2000 km. In the first step ofevent selection we chose the strongest events that occurred within last years. We have
chosen to use fairly recent events since this will enable us to use data recorded bystations of the National Swedish Seismic Network (SNSN), a dense regional seismicnetwork of broadband stations allowing good quality records of local events (e.g. Ols-son, 2007). The number of SNSN stations has grown rapidly in the last decade andtoday has a good coverage of most of Sweden. Because of the size of the Baltic Shield5
(about 1500 km in diameter) we limited out data set to events from the outskirts of theBaltic Shield in order to obtain large enough station to event distance range. The cho-sen 9 events are listed in Table 1 and shown in Fig. 2 (event no. 8 at Novaya Zemlya islocated outside of the map frame). The magnitudes of events are in the range 2.7–5.9.Exception is the small event no. 10 in Western Finland of magnitude 1.1 which occurred10
nearly at the same time as event no. 9 from Rhine Graben (both events were recordedsimultaneously). In Fig. 2 earthquakes are marked by black stars with numbers andseismic stations providing data are marked by black dots.
4 Average velocities, reference model and time corrections
The relatively big number of seismic stations in the Baltic Shield permits for relatively15
dense recordings of events, with a distance between stations of 30–100 km. This spac-ing is sufficient in lithospheric scale studies where the penetration depth of body wavesis a few hundreds of km and the wave length is of the order of a few km. Recordingstations are not linearly aligned which would permit for easy 2-D interpretation of thestructure along profile, but scattered in a wide corridor of a few hundreds km width.20
This means that two stations at similar epicentral distance may lie in locations withsignificantly different altitude and Moho depth. For modelling the lower lithosphere andasthenosphere along profiles, the original data needs to be time corrected for topog-raphy and the Moho depth for each event and each station location. Topography forseismic station locations is changing from close to sea level up to 630 m (see Fig. 2).25
Differences in the Moho depth in the study area are significantly larger. As seenfrom Fig. 3 the Moho depth in the Baltic Shield is changing in range of 30–60 km
(Grad et al., 2009). An average Moho depth was calculated for the area within theyellow frame shown in Fig. 3 using data from the digital Moho depth map by Gradet al. (2009). The average depth is 45.7 km, and in reference model 46 km depth wasused. P wave velocities in the reference model were compiled from seismic studies inthe area. In Fig. 3 the analysed profiles are shown by white lines (Hirschleber et al.,5
1975; Lund, 1979; Guggisberg, 1986; Luosto, 1986; Grad and Luosto, 1987; Luostoet al., 1989, 1990, 1994; Grad et al., 1991; FENNIA Working Group, 1998; Uski et al.,2012; Tiira et al., 2013) and places of sampled velocities are shown by white dots.All velocity-depth relations were sampled at 1 km depth intervals. The velocity valueswere weighted according to reliability of the models. The highest weight was given to10
data from modern refraction and wide-angle reflection profiles with a dense system ofobservations and good reciprocal coverage (e.g. SVEKA, BALTIC, POLAR). Velocityin the crust and uppermost mantle were directly extracted from 2-D numerical mod-els with a spacing of 60 km along profiles. Only parts of models sufficiently sampledby rays were taken to velocity-depth analysis. For older profiles with a sparse system15
of observations published 1-D models were adopted (e.g. Blue Road, FENNOLORA,Sylen-Porvoo).
P wave velocities in the crust and uppermost mantle of the Baltic Shield were sam-pled in 67 locations (white dots in Fig. 3). Comparison with location of seismic stations(Fig. 2) shows a good coverage for the whole area of the Baltic Shield. In total 426920
values of velocity were used for determination of average velocity-depth relations in thecrust and uppermost mantle. The data were fitted by linear functions:
V (z) = 5.97+0.0257z for the crust (1)
and
V (z) = 7.71+0.0091z for the uppermost mantle (2)25
where V is P wave velocity in km s−1 and z is depth in km. The data distribution isshown in gray scale in Fig. 4a. Apart from fitted relations (1) and (2) the reference
model is shown by thick black dotted line (Fig. 4a). The model has a 46 km thick crustwith velocities increasing from 5.97 at the Earth’s surface to 7.24 kms−1 at the 46 kmdepth. The velocity below the Moho at 46 km depth is 8.13 kms−1.
Traveltimes for reference model with 46 km Moho depth are shown by the black linein Fig. 4b. For comparison two traveltime curves for shallower Moho (34 km depth,5
blue line) and deeper Moho (58 km depth, red line) are shown. The differences of timeare about ±2 s at distance 300–500 km, decreasing to about ±1 s at distance 1000–1500 km. Traveltimes were calculated with 1 km interval for the Moho depths from 34to 58 km. The time corrections were calculated using interpolation for both distancesand Moho depths below event and station. The final time corrections included also10
correction for topography. An example of the application of time corrections is shownin Fig. 5. The map in Fig. 5a shows the location of event no. 3 (Lubin; marked by blackstar) and seismic stations (black dots) with their numbers. Corresponding sections withcorrections and without corrections are shown in Fig. 5b and c, respectively. Red dotsare picks of first arrivals (the same picks are shown in both sections). Red lines are15
traveltimes and gray bands show scattering of the data. Application of time correctionsimproves correlation. The scattering of first arrivals is about 1 s for the section withtime corrections (Fig. 5b), and about 2 s for the section without corrections (Fig. 5c).The histogram in Fig. 5d shows a distribution of 604 total time corrections. About 93 %of the corrections are in the range from −0.1 s to 0.8 s, with maximum for 0.2–0.5 s20
interval (282 corrections). Such a processing was applied for all sections in this paper,which permits us to use a reference crustal model, without topography (0 a.s.l.) andwith a reference Moho at 46 km depth to model our data. After applying the correctionsall sections correspond to Moho depth of 46 km and stations at sea level.
5 Searching for asthenosphere beneath the Baltic Shield25
To study the lithosphere-asthenosphere system of the Baltic Shield we chose 9 events(Table 1, Fig. 2) from outskirts of shield. For all record sections time corrections were
applied as described in the previous section. Poor quality and noisy seismograms wereremoved from the sections. Some seismograms were also omitted to avoid crowding ofseismograms from stations with similar epicentral distances. All record sections werenormalized, filtered, and drawn with reduction velocity 8 kms−1 (in some cases withreduction velocity 4.5 kms−1 for sections of S waves).5
Modelling for all sections in this paper was done using the ray tracing technique andsoftware SEIS83 (Červený and Pšenčík, 1984). As a reference crustal model for Pwave velocity we used the model described in the previous section. Crustal S wavevelocities were recalculated from P velocities using the relation Vp/Vs = 1.67 in theuppermost crystalline basement and Vp/Vs = 1.77 in the lower crust (see e.g. Grad10
and Luosto, 1987; Bogdanova et al., 2006; Uski et al., 2012). The models of the mantlestructure were derived by trial-and-error forward modelling. Traveltimes and syntheticseismograms were calculated for 1-D models with Earth-flattening transformation forwaves from a point source (Hill, 1972).
Two examples of record sections for event no. 3 (Lubin) and for event no. 6 (Kola) are15
shown in Figs. 6 and 7. Apart of the full wave fields of P, S and surface waves (Figs. 6aand 7a) enlarged parts of sections are shown for S (Figs. 6b and 7b) and P waves(Figs. 6c and 7c). Red dots are picked first arrivals for P and S waves, and red lines arefirst arrivals of P and S waves calculated for the iasp91 model (Kennett and Engdahl,1991). Although the crustal thickness in iasp91 model is only 35 km (in comparison20
to 46 km for the Baltic Shield), observed P and S arrivals are both significantly earlierthan predicted by the iasp91 model – at distance 1700 km about 3–4 s for P waves andabout 6–8 s for S waves. This is a clear indication that lithospheric P and S velocitiesbeneath the Baltic Shield are higher than those in the iasp91 model. Also the observedcontinuation of the first arrivals up to 1700–2000 km distance could be interpreted as25
a lack of a “shadow zone”. High lithospheric velocities for the East European Cratonusing Pn and Sn waves recorded at teleseismic distances were described nearly fiftyyears ago by Båth (1966).
Sections with records up to 2200–3000 km distance are shown in Figs. 8 and 9.For event no. 8 (Novaya Zemlya) good quality first arrivals of P waves are recorded inthe distance range 1300–3000 km. At an epicentral distance of about 1800 km a clear“shadow zone” is visible (marked in Fig. 8 by arrow). Corresponding rays are shownin ray diagram: red rays traveling in the lithosphere and navy blue rays reflected from5
the bottom of the asthenosphere and mantle including “410” and “660” km boundaries.Also shown in Fig. 8 is a comparison between the 1-D velocity model for this event andthe iasp91 model. An interesting observation is related with frequency of first arrivals.Lithospheric phases have significantly higher frequencies than deeper phases. Thischange occurs at relatively short epicentral distance – compare them at 1900 km and10
2100 km. One explanation could be stronger attenuation of high frequency content ofa pulse traveling trough the asthenosphere. In such a case the asthenosphere shouldbe characterized by a lower value of the quality factor Qp. The next figure shows nexttwo sections (Fig. 9). For event no. 2 (Spitsbergen) good quality P waves are recordedin the distance range 800–2400 km. First arrivals are far away from explosive character15
– monotonic increase of amplitude to maximum of the seismic moment takes about5 s (Pirli et al., 2010). In such a case correlation of further arrivals is practically im-possible. This is illustrated in Fig. 9b showing synthetic seismograms calculated for anextremely long source pulse. However, because of the big size of event (M = 5.9) cor-relation of first arrivals was unquestionable and was done through a manual process on20
a computer screen, using software ZPLOT (Zelt 1994) allowing flexible use of scaling,zooming, filtering, and reduction velocity. For the two northern events at Spitsbergenand Novaya Zemlya we observe low velocity layer – asthenosphere. The LAB beneathBarents Sea was found there at depth of about 200 km (198 km and 201 km for eventno. 2 at Spitsbergen and for event no. 8 at Novaya Zemlya, respectively).25
Section for event no. 7 (Bełchatów) shows continuous first arrivals of P waves withno evidence for a “shadow zone” in the whole range of registration (Fig. 9c). A lack of“shadow zone” could be interpreted as a lack of asthenosphere beneath central part ofthe Baltic Shield, or that LAB in this area occurs deeper (> 200 km).
Other data we collected do not show evidence for the existence of asthenosphere be-neath the Baltic Shield. However, they contain information about the lower lithosphereinhomogeneities which could be deduced from P and S waves (Figs. 10 to 13).
The sections shown in Fig. 10 need more explanation. About 4.5 min after event5
no. 9 (Rhein Graben, M = 4.0) at a distance of about 1800 km towards the norheast,a smaller earthquake occurred in Western Finland (event no. 10, M = 1.1; see Table 1).The section in Fig. 10a shows both events, recorded simultaneously. Phases attributedto the small event in Finland are here highlighted in yellow (recorded in epicentraldistance 1500–2200 km of event no. 9). In Figs. 10b and c the location of this event10
and record section are shown. Pg, Pn, Sg and Sn waves calculated for the referencemodel of the Baltic Shield quite well fit observations. For event no. 9 (Fig. 10a) strongP and S waves are recorded up to 1500 km distance. However, in the distance range1600–2300 km the strong P waves have no corresponding strong S waves. This factis difficult to interpret as lowering of S wave velocity (S wave asthenosphere). The15
explanation could be lower quality factor for S waves (“Qs asthenosphere”?).Lower lithospheric inhomogeneities were interpreted beneath southern Sweden us-
ing data for event no. 1 (Skagerak). At a distance of around 600 km a small break in thecontinuity of the first arrivals of P waves is observed (Fig. 11). This can be explainedby a model with a 16 km thick LVL 15 km below the Moho with a P wave velocity drop20
of −0.33 kms−1. Such a velocity contrast is sufficient to explain the strong amplitudesseen in the distance range 1600–1800 km, as multireflections within the LVL. Red linesin Fig. 11 show first arrivals (Pn and P), reflection from the bottom of LVL (1) andtraveltimes of five multiples in LVL (2)–(6). In synthetic seismograms reflection fromthe bottom of LVL (1) and multiples (2) and (3) are strong, (4) are weak, and higher25
multiples practically invisible. We do not interpret the LVL as asthenosphere but as aninhomogeneity in the lower lithosphere.
The next record section (Fig. 12a) illustrates a differentiation of the upper mantlevelocities observed for the event no. 4 (Varangerfjord). Although time corrections havebeen applied, considerable scattering of first arrivals is observed, particularly in thedistance interval 200–600 km. Late arrivals are marked in green and correspondingstations are also marked in green in neighboring map (Fig. 12b). Low velocities in the5
lower lithosphere coincide with the northern part of the Baltic Shield and the Cale-donides, and “normal velocities” with the area towards the south (red dots). Even morescattering of first arrivals, up to about 3 s at about 800 km distance, is observed forevent no. 5 (Scania) shown in Fig. 13. In this case first arrival picks were split into threegroups: normal arrivals to the north (red dots), faster to the northeast (light blue dots),10
and the fastest to the east (navy blue dots).In the last two sections for event no. 4 (Varangerfjord) and for event no. 5 (Scania)
thick gray bar highlights far distance Pg waves recorded up to about 700 km distance(Figs. 12 and 13). It means, qualitatively, that attenuation of P waves in the Baltic Shieldcrust is relatively low. Beneath the SVEKA profile in Finland (Grad and Luosto, 1994)15
Qp-factor in the uppermost 1 km is 50–80 only, but in the crystalline crust it reach valuesof 500–800, which means that attenuation in the crust is extremely low.
The records in the last two sections (Figs. 12 and 13) reach distance up to about1700 km, too short to reach LAB in central part of the Baltic Shield, where it is at depthdeeper than 200 km. On the other hand both events, located in the north and in the20
south of the shield, give valuable information about the lower lithosphere – lid. Basedon these data the Baltic Shield lid can be divided into areas of different P wave velocity(Fig. 14a): normal velocity (approximately corresponding to the area of Sweden), fastervelocities (central and southern Finland), the fastest velocities (at southeastern edge ofshield), and the lowest velocities (coincides with the northern part of the Baltic Shield25
and the Caledonides in northern Finland and northern Norway).
The seismic structure of the lithosphere–asthenosphere system beneath the BalticShield was studied using local earthquakes with magnitudes in the range 2.7–5.9.Good quality records up to 2200–3000 km distance were obtained by relatively denselylocated seismic stations with spacing in the distance range 30–100 km. All the data5
were corrected for topography and the Moho depth for each event and each station lo-cation, using reference model of the Baltic Shield with 46 km thick crust. A summary ofthe Baltic Shield structure is compiled in Fig. 14, showing a map of velocity provinces(Fig. 14a), P wave velocities in lid (Fig. 14b), and P and S wave velocities in the uppermantle (Fig. 14c).10
In general, P and S arrivals observed in the Baltic Shield are significantly earlier thanpredicted by the iasp91 global velocity model, which clearly indicates that lithosphericP and S velocities of the Baltic Shield are higher (see Figs. 6b, c and 7b, c).
For the two northern events (Spitsbergen and Novaya Zemlya) we observe a LVL thatwe interpret as the asthenosphere. The LAB beneath Barents Sea was found at depth15
198 km and 201 km for event no. 2 (Spitsbergen) and for event no. 8 (Novaya Zemlya),respectively. The location of deepest points reached by rays in the lithosphere areshown in the map (Fig. 14a) by two red circles, and red dotted line shows approximately200 km isoline of the LAB depth. Other data show that LAB in the central Baltic Shield,if it exists, should be deeper than 200–250 km. These depths coincide with results of20
surface wave tomography of the Barents Sea and surrounding regions (Levshin et al.,2007), as well as thermal modelling of the East European Craton (Artemieva 2003,2007). The asthenosphere thickness for events at Spitsbergen and Novaya Zemlyawere determined only from P waves, and they are 69 and 61 km, respectively (seegray lines in Fig. 14c). Analysis of the other record sections show that first arrivals of P25
waves are continuous in the whole range of registration, while corresponding S wavesare weaker. This can be explained by lowering of S wave velocity or decrease of thequality factor for S waves (S wave asthenosphere?).
From analysis of record sections from different areas in the Baltic Shield, the lid ofthe shield can be divided into areas of different P wave velocity. In Fig. 14a the areaof normal velocity is marked in red (approximately corresponding to the area of Swe-den), area of faster velocities is marked in blue (central and southern Finland), area ofthe fastest velocities is marked in navy blue (at the southeastern edge of shield), and5
the area of the lowest velocities is marked in green (coincides with the northern partof the Baltic Shield and the Caledonides in northern Finland and northern Norway).The corresponding models of P wave velocity are shown in the same colors in Fig. 14.Differentiation of the velocity could be interpreted as inhomogeneity of the lid. How-ever, another interpretation could be anisotropy of the lithosphere, with fast velocity10
direction close to east-west, and slow velocity direction close to south-north. Recently,Eken et al. (2012) interpreted distinct differences in tomographic inversions of SV- andSH wave traveltimes as associated with anisotropy of the lithospheric mantle downto depths of about 200 km. Amplitudes of the velocity perturbations decrease below∼ 200 km, that is sub-lithospheric mantle (asthenosphere?).15
Apart of regional velocity differentiation we found small scale lithospheric inhomo-geneities. Beneath southern Sweden, 15 km below the Moho, a LVL of 16 km thicknesswith a drop of velocity for P waves −0.33 kms−1 was found. Reflected waves and mul-tiples in the LVL agree well with observations (Fig. 11). For the same area of southernSweden at a similar depth a lithospheric reflector was found from FENNOLORA data20
(e.g. Guggisberg 1986), as well as in the uppermost mantle beneath southern Finlandfrom local events registered by the SVEKALAPKO seismic array (Yliniemi et al., 2004).
8 Conclusions
– The asthenosphere was found in the area north of the Baltic Shield (beneath theBarents Sea) at depth of about 200 km.25
– No evidence was found for the asthenosphere beneath the Baltic Shield – it hasto be deeper than 200 km if it exists at all. Even if it exists deeper it could not bedetected by “refraction” method. For distance larger than about 2000 km a shadowzone in the first arrivals would be masked by deeper waves from the “410” and“660” km boundaries.5
– Difficulties in the interpretation of next arrivals result from the complicated sourcefunction of natural earthquakes (e.g. event no. 2) or from multireflections in theLVL (event no. 1). Interpretation can also be complicated by seismic noise whensignals from distant earthquakes are not strong enough, or obscured by simulta-neous recording of small local events (see event no. 9 and event no. 10).10
– Differentiation of the velocity in the lid could be interpreted as regional inhomo-geneity or anisotropy, however it needs more studies using 3-D tomography.
– Answer for the question in the title: “LAB or LID?” – LID beneath the Baltic Shield!
Acknowledgements. The authors wish to thank Finnish Academy of Science and Letters,Väisälä Foundation for financial support. This work was partially supported by NCN grants15
UMO-2011/01/B/ST10/06653 and DEC-2011/02/A/ST10/00284. We are also grateful to thestaff at the Swedish National Seismic Network (SNSN) for giving us access to their data. Wave-form data from seismic stations operated by the Institute of Seismology of the University ofHelsinki, the Sodankylä Geophysical Observatory of the University of Oulu and from the Na-tional Swedish Seismic Network (SNSN) were used in this study. For specific events data from20
Norwegian and Russian stations were made available by the Bergen Seismological Observa-tory and NORSAR in Norway. The authors wish to thank GEOFON, GFZ German ResearchCentre for Geosciences for earthquake information and waveform data. Geographic data han-dling and plotting was done with GMT software by P. Wessel and W.H.F Smith (Wessel andSmith 1991, 1998).25
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Table 1. Events used for lithospheric structure studies in the Baltic Shield area data compiledfrom Helsinki catolog Uski and Raime (2010), Pirli et al. (2010) and Wiejacz and Rudziński(2010).
event no. date yyyy-mm-dd time hh:mm:ss lat φ [◦ N] long λ [◦ E] depth [km] M place
Fig. 1. The seismicity map of the Central and Northern Europe (January 2008–September2011) showing location of 2698 events: 2285 from Helsinki catalog (Uski and Raime, 2010)and 413 from GEOFON. Events are mostly shallow: 90 % of events have depth less than 20 km,maximum depth is 70 km for event in Spitsbergen; the highest magnitude for an event is 5.9.
Fig. 2. The location of nine events (numbered black stars) and seismic stations (dots) usedin this paper for the study of the lithosphere-asthenosphere system beneath the Baltic Shield,overlain on the topography/bathymetry map of the region (Amante and Eakins, 2009). The bluestar with number 10 shows the location of a small event which occured in Western Finlad andwas recorded together with event no. 9. For more details see Table 1. TESZ – Trans Europeansuture zone, SW margin of the East European platform; VTE – Variscan terranes of Europe;KR and MR – Knipovich and Mohns ridges, parts of Mid-Atlantic Ridge; COT – continent-oceantransition.
Fig. 3. Moho depth map of the European Plate (Grad et al., 2009). Also shown are the seismicrefraction profiles (white lines) with the positions (white dots) where velocities were sampledto compile the average P wave velocity model of the crust and uppermost mantle of the BalticShield. The yellow frame shows the area for which an average Moho depth (45.7 km) wascalculated. Data were compiled from Hirschleber et al. (1975); Lund (1979); Guggisberg (1986);Luosto (1986); Grad and Luosto (1987); Luosto et al. (1989, 1990, 1994); Grad et al. (1991);FENNIA Working Group (1998); Uski et al. (2012); Tiira et al. (2013).
Fig. 4. (a) Average P wave velocities of the crust and uppermost mantle of the Baltic Shieldfitted by the linear functions (1) and (2) given in the text. In total 4269 values of velocity in67 locations were used (see white dots in Fig. 3). The gray scale shows the density of data in0.1 km−1 1×1km cells. The reference model shown by the black dotted line has 46 km thick crustwith velocities increasing from 5.97 at the Earth’s surface to 7.24 kms−1 at the 46 km depth. Thevelocity below the Moho at 46 km depth is 8.13 kms−1. (b) A comparison of traveltime curvesfor the reference model (black line for Moho depth 46 km) with traveltime curves for shallowerMoho (34 km depth, blue line) and deeper Moho (58 km depth, red line). Reduction velocity is8 kms−1. For more explanation see text.
Fig. 5. Example of application of the time correction discussed in the text. (a) Map showingthe location of event no. 9 (Rhine Graben) marked by star and seismic stations in southernSweden (numbered dots) used in this comparison. (b) Recorded seismograms with time cor-rections applied. Note much lower scattering of first arrival times in the corrected section c. (c)Recorded seismograms without time corrections. Red dots are picked first arrivals for P waves,and thick gray line shows a scattering range of first arrival times; reduction velocity 8 kms−1.(d) Histogram showing distribution of 604 total time corrections. About 93 % of corrections arein range from −0.1 to 0.8 s, with maximum for 0.2–0.5 s interval (282 corrections).
Fig. 6. Examples of record sections with time corrected seismograms for event no. 3 (Lubin).(a) Full wave field of P, S and surface waves. Filtration 1–7 Hz, normalized traces, reductionvelocity 8 kms−1. Red lines are first arrivals of P and S waves calculated for the iasp91 model.Enlarged parts of this section are shown for P and S waves. (b) Section for S waves, filtration2–7 Hz, normalized traces, reduction velocity 4.5 kms−1. (c) Section for P waves, filtration 2–10 Hz, normalized traces, reduction velocity 8 kms−1. Red dots are the picked first arrivals forP and S waves, and red lines are the first arrivals of P and S waves calculated for the iasp91model. Phases related to the “410” and “660” km boundaries are also shown in the sections.Note the early arrivals of both P and S waves compared to the iasp91 model – at distance1700 km about 4 s for P waves and about 8 s for S waves.
Fig. 7. Examples of record sections with time corrected seismograms for event no. 6 (Kola). (a)Full wave field of P, S and surface waves. Filtration 1–7 Hz, normalized traces, reduction velocity8 kms−1. Red lines are the first arrivals of P and S waves calculated for the iasp91 model.Enlarged parts of this section are shown for P and S waves. (b) Section for S waves, filtration2–7 Hz, normalized traces, reduction velocity 4.5 kms−1. (c) Section for P waves, filtration 2–10 Hz, normalized traces, reduction velocity 8 kms−1. Red dots are the picked first arrivals forP and S waves, and red lines are the first arrivals of P and S waves calculated for the iasp91model. Phases from “410” and “660” km boundaries are also shown. Note the early arrivals ofboth P and S waves compared to iasp91 model – at distance 1700 km about 3 s for P wavesand about 6 s for S waves.
Fig. 8. Record section for event no. 8 (Novaya Zemlya). Filtration 1–5 Hz, normalized traces,reduction velocity 8 kms−1. The beginning of the shadow zone of the first arrivals is observedat a distance of around 1900 km (shown by arrow), and the corresponding depth of the lowvelocity zone (LAB) is 201 km. Red dots are picked the first arrivals of P waves, and red linesare traveltimes. The Vp model (black line) is shown together with the iasp91 model (yellow line).The bottom left plot shows rays in the mantle (red in the lithosphere and navy blue in deepermantle).
Fig. 9. Record sections of two events (with and without asthenosphere), together with trav-eltimes of the best fitted models. (a) Section of event no. 2 (Spitsbegen). Filtration 2–15 Hz,normalized traces, reduction velocity 8 kms−1. The beginning of the shadow zone of the first ar-rivals (shown by arrow) is observed at distance of about 1800 km, and corresponds to the depthof the low velocity zone (LAB) at 198 km. (b) Synthetic seismograms for event no. 2 (Spitsbe-gen). For synthetics a complex source was simulated with a length of radiation of around 20 s.The figure shows the difficulties in interpretation of secondary arrivals masked by long sourcefunction. However, the shadow zone in first arrivals is still clear, which permits conclusionsabout the LVL – asthenosphere. (c) Section for event no. 7 (Bełchatów). Filtration 2–15 Hz, nor-malized traces, reduction velocity 8 kms−1. A continuity of the first arrivals is observed in thewhole distance range up to about 2000 km. In this case the low velocity zone (asthenosphere)has to be deeper than 180–200 km, if it exists at all. In all sections red dots are picked firstarrivals of P waves, and red lines are fitted traveltimes.
Fig. 10. Record section (a) for event no. 9 (Rhine Graben) with signals from simultaneous smallevent in Finland (no.10; highlighted by yellow). (b) Location of event 10 and seismic stationsrecording this event. (c) Record section of event 10. Note the good fit of P and S traveltimescalculated for the reference model.
Fig. 11. Record section (bottom) for event no. 1 (Skagerak) with synthetic seismograms (middleand top). Note in the section a shadow zone (at distance of around 600 km) which is an effectof a LVL in the lower lithosphere (at depth interval 61–77 km). Red lines show the first arrivals(Pn and P), reflections from the bottom of the LVL (1) and traveltimes of five multiples in theLVL (2)–(6). Multiples in the LVL (2)–(4) explain the strong amplitudes in further arrivals in thedistance range 600–800 km.
Fig. 12. (a) Differentiation of the upper mantle velocities observed for event no. 4 (Varanger-fjord). Low velocities in the northern part of the Baltic Shield and the Caledonides (green dots),and normal to the south (red dots). Thick gray bar highlights far distance Pg waves recordedup to about 600 km distance. (b) Location map showing recording stations in correspondingcolours.
Fig. 13. (a) Differentiation of the upper mantle velocities observed for event no. 5 (Scania).Normal velocities to the north (red dots), higher to NE (light blue dots), highest to the east(navy blue dots). Thick gray bar highlights far distance Pg waves recorded up to about 600 kmdistance. (b) Location map showing recording stations in corresponding colours.
Fig. 14. Summary of the Baltic Shield structure. (a) Map showing regions of lithospheric mantlevelocities: low in the Caledonides and the northern Baltic Shield (green), normal (red), higher(light blue) and highest in the east (navy blue dots). Two red circles in the north of the shieldshow locations of deepest rays from event no. 2 (Spitsbergen) and event no. 8 (Novaya Zemlya)corresponding to a LAB at 200 km depth in the Barents Sea. LAB in the central Baltic Shieldshould be deeper than 200–250 km. (b) Models of lithospheric mantle velocities for P wavesfor events no. 4 (Varangerfjord, green line) and event no. 5 (Scania, all other lines). Colorscorrespond to those on adjacent map; iasp91 model (yellow line) is shown for comparison. (c)P and S wave velocity models for the Baltic Shield together with the iasp91 model (yellow line).Two gray lines for events no. 2 and 8 document LAB and asthenosphere. The lowest velocitieswere found for event no. 4 in the Caledonides and in the northern Baltic Shield. Model for eventno. 1 shows a low velocity zone in the lower lithosphere of the southern Baltic Shield. See textfor further discussion.
